1. CHANNEL TUNNEL
TUNNELS CONSTRUCTION
Author Pierre-Jean Pompée
Scope
Within the 5 years following contract award,
3 parallel tunnels, 50.5 km each, had to be
driven and lined with high-strengh precast
segments : 38 km undersea, 3.2 underland
in France, 9.3 km underland in UK. 2/3 of
the total had to be excavated from UK side,
and 1/3 from French side where ground
conditions were unfavourable. 6 drives
(total 12), 3 to the sea and 3 to the land
have been conducted from each of the two
tunneling sites, located as close as possible
to the sea : Shakespeare Cliff (west of
Dover) on the UK side, and Sangatte (west
of Calais) in France.
The service tunnel (ST) has a internal
diameter of 4.8 m (excavated diam.5.8 m).
The two running tunnels (RT) are at 30 m
distance, lined diameter is 7.6 m
(excavated diam.8.8 m). They are
connected together by 194 piston relief
ducts (every 250 m, diam 2 m), and by 270
cross passages (every 375 m) and 210
technical rooms (all diam 3.3 m) to the
service tunnel, requiring special cast iron
segments at each junction.
Geology and alignment
Many geotechnical investigations have
been carried out across the Channel since
1875, including over 100 boreholes. Total
information available including final
investigation after contract award and
geophysical survey undertaken in 1988,
(specifically for crossing beneath the Fosse
Dangeard and crossovers sites),comprised:
- 120 marine boreholes and about 1,000
km of geophysical (seismic reflection),
traverse lines,
- adits and short lenght of tunnels
excavated during previous attempts.
Optimum tunnelling conditions are within
the Chalk Marl, clayey carbonate
mudstone, weak (20 to 200 bars),
homogeneous rock, believed to be mainly
impermeable, with any water restricted to
discontinuities such as faults or fissures
concentrated on French side. It is a
tunnelling medium adequate for TBMs.
High precision alignment
The tunnel alignment was a critical stage in
the design process. It was later optimised
using experience gained while driving the
service tunnel itself, and design
developments at the crossover and
pumping stations, avoiding fault zones
which would reduce tunnelling progress.
Geometrical criterias applicable to a high-
Above : View of a running tunnel on French side at the beginning of the tunneling phase (1988), showing temporary
fixed equipments (water pipes, ventilation duct diam 1600mm, power and radio communications cables, double track
and walways...). Below : Tunneling progress on 29th of April 1990, 7 months before first undersea breakthrough,
showing forecasted ("M"), and final (black balls) meeting points.
speed railway impose large radius curves,
leading to drive 150 km of tunnels within
150 mm tolerances: 40 mm for
topography, 55 for TBM steering, 25 for
segments manufacturing/erection (ring
axis), 30 between surfaces of adjacent
segments. TBMs, launched over 38 km
distance, had to meet with precision !
Topographical survey
Different systems in UK and France
British maps are based on Mercator
system, French maps on Lambert system.
Sea level on both sides differed by 440 mm
according to a 1958 study, reduced at 300±
80 mm in 1987, measured at 360 mm after
breakthrough.
Joint mapping and levelling
Mercator system was adopted, but based
on the mid-Channel 1°30 East meridian.
Satellite Global Positioning System (GPS,
precision 5 cm for 50 km) was used and
water movement studies reduced levelling
uncertainty , leading to the RTM87 grid,
and the NTM88 levelling ("Reseau" and
"Niveau TransManche"). Final orientation
to the North required a gyrotheodolite with
precise astronomical calibration and
correction of all horizontal angles, locally
different due to the Mercator system.
Survey brough underground
RTM87 was brought underground trough
the inclined adits of the UK side and the
Sangatte shaft in France through
benchmarks set at the tunnels starting
points with a precision of 0.6 mm (5 mm
error would have meant 1.5 m at 15 km
distance). Grid was relayed along TBMs
journey with computer correction of
temperature, pressure, hygrometry. After
first breakthrough, a reference point was at
least available for running tunnels survey.

2. Geological profile and description of the works. Tunnel slope is between 0.2% and 1.1 %, often found at 0.6% on UK side and at 1.1% on French side.
Breakthrough actual precision
Once the marine service tunnel TBMs
stopped at 100 m distance, measured
lateral offset was 358 mm; levelling : 58
mm; chainage :75 mm. Readjustment over
the final 50 m, hand excavated, produced a
smooth junctioning of the two drives.
Running tunnels all came out within the 15
cm tolerance.Underland TBMs came out at
less than 2 cm from the original alignment.
Tunnel Boring Machines
Design criteria on French side
Critical programme and unfavourable
geology determined a whole tunnelling
system (TBMs, back-up systems, linings,
and overall logistics). Design criteria were:
- progress without ground treatment in
watery conditions in France, especially
the first 4 km undersea (where the chalk
is fractured), and 3.2 km underland,
(below water table).
- progress at high speed (4.4 lm/h) in
dry, fair to good, ground conditions.
- ensure safety and reliability for a
highly complex, massive, tunnelling
logistics designed for peak boring
rates. A huge, waterproof, shaft housed
all French tunnels logistics at Sangatte.
TBMs description
11 TBMs were ordered by TML. On the UK
side, 6 open, full-face TBMs (3 undersea
and 3 underland) were used in conjunction
with unbolted, expanded precast concrete
linings by use of wedge-shaped key.
The 5 French TBMs have been launched
from the Sangatte Shaft: 3 undersea, 2
underland, one being turned back to drive
the north running tunel underland. There
are full face, earth-pressure type, and allow
erection of watertight precast segments
inside the shield concurrently with face
excavation. Their unique design combines
technologies for both soft ground
underwater and hard rock.
The 3 undersea French TBMs include
articulated double shield operating in closed
mode in wet zones (minimum rate of 3
m/h), or in open mode at high speed in dry
zones. Cutter chambers are sealed to
withstand a hydrostatic pressure of 11 bars:
excavated chalk had to pass through a twostage pressure lock before being ejected at
atmospheric pressure into the muck
wagons. Containment is achieved at the
end of the spoil extraction screw either
through a dual discharge pump system or
through a second screw creating a plug.
The 2 underland French TBMs included a
single shield, Archimedes screw
containment and a ″casing rotator″, to
withstand a hydrostatic pressure of 3 bars.
French TBMs guidance system
The ZED 260 guidance system used a laser
beam, set up to a predetermined alignment,
and received on a screen on the cutting
head. After analisis, an on-board computer
displayed, in the cab, differences betwwen
designed and actual TBM's 3 dimensional
position, allowing the driver to correct by
steering to zero. Steering was achieved by
selection of the push rams housed in the
telescopic section.
Real time monitoring in the cab of
complete operational status of the TBM
T3 process was ensured by an IBM 7532
superviser: pressures, speed, stroke of
hydraulic jacks, spoil rotating screws and
tail seals, but also electrical intensity and
instant power of the 12 motors, water
injected flows - 20 bars and 30 bars - and
earthpressure at 5 different locations of
cutting chamber, plus rotating head
position. Programmable controlers
managed overall process safety and failure
detection (ALSPA C350, 2000 I/O), grout
injection (SIEMENS 115V), and segments
feeder (TELEMECANIQUE TSX47-20).
Main data were plotted and/or transmitted
to surface. Long term tracing of segments
rings main data was automatically
available. Video screens showed conveyors
operations.
Probes, ground treatment
Site investigation boreholes covered a grid
of 1 km width only, leading to use the
Left : Probes on French side around the T1 (marine Service tunnel TBM). Below : Description of the TBM T3 (Marine
running tunnel south, french side). it was followed by a 16 cars, 250 m long, backup system.

3. Above, top : Tunneling programme on UK side (left) and
French side (right). Above image : Inside the control cab
of TBM T3 (French marine south running tunnel). Right
image : TBM T3 during preassembly at the factory.
Bottom table, right : TBMs data on French side.
service tunnel as a pilot tunnel by probing
ahead of the TBM, to locate: water bearing
fissures, site investigation boreholes, deep
weathering, and proximity of Grey
Chalk/Gault Clay/Fosse Dangeard fault.
On the UK side, grout injections through
holes drilled from the marine service tunnel
improved conditions ahead of running
tunnels TBMs, producing a 3 m annulus of
treated ground at the upper half of both
tunnels. It had to be carried out with
minimum interferences. It proved to be
successful since the average rates
increased from 85 m/week to 210 m/week
in the treated ground.
In France, overlaping destructive probe
holes were drilled from the face of T1
(marine service tunnel), to give advance
warning of unstable ground and high or
saline water inflows. Micropalaeontological
examination of the returns was used to
determine the level of the Chalk Marl.
Detailed geotechnical logs were made of
the side walls and (where possible) at the
front. The water inflow prooved to be
considerably lower than the specified
values (7 l/s/km for RTs, 6 l/s/km for ST).
High boring rates
The best rates achieved in 30 consecutive
days, were, in dry ground: 1,716 m (UK
land south running tunnel), and in
waterbearing ground: 1,232 m (French
south running tunnel).
Innovating approach on French Side
Many refinements have been found, in
positioning of cutting-head tools as well as
in operating the machines. Anticipated
progress rates have been considerably
exceeded in France, leading to increase the
original excavation length by 8125 m.
TBM T4 (French land service tunnel)
achieved 837 m in the waterbearing zone
under a pressure of 3 bar. TBM T1 (marine

4. Above : "Pinched" rings on French side allow to bolt and seal the rings, and contribute to TBM steering : while
following curves, rings are kept in contact. Straight drive, vertical or horizontal curves are achieved by rotation of the
ring around tunnel axis (for example, red "key" segments are not in the same position than the blue ones).
Right : Service tunnel with temporary tunneling fixed equipment on UK side (above) compared with French side
(below, and also cover page). In UK, the temporary track is ballasted;the key segment of each ring is at a fixed
location, and lining is unbolted. In France, temporary track is installed on metal frame; the key segment is at different
locations on two adjacent rings; bolt boxes are visible on each segment.
service tunnel), T2 and T3 (marine running
tunnels) reached a final daily rate over 50
m, following a slow start of T1 in difficult
ground, but using the experience gained in
the service tunnel to optimize T2 and T3.
Air cushions allowed to turn the land TBM
T5, renamed T6, back to Sangatte.Only 16
Tonnes of traction were needed instead of
dismantling/displacing a 600 T TBM.
Lining
Lining was the critical operation that
dictated tunnelling progress.
UK side:
Service Tunnel machines were designed for
a cycle of 18 min, and 22.5 min for running
tunnel machines. The lining was a precast
concrete unbolted wedge block with pads
on the extrados to allow for a 20 mm grout
layer. There were 7 segments per ring in
the service tunnel and 9 segments per ring,
all different, in the running tunnels, with a
thickness varying depending of the
overburden, from 54 cm (land), to 27 cm
(sea, from 0 to 7 km to the shore) or 36 cm
(sea, over 7 km from the shore). The
placing of segments was considerably
altered when the stability of the ground was
not adequate.
French side:
The main design constraint was
waterproofing with hydrostatic water
pressure up to 11 bars. Complex numerical
models were used - in addition to the
convergence-confinement method - to
evaluate the interaction between tunnels,
the influence of the proximity of a Gault
Clay layer, and the quality of grouting
behind the segments. Optimization studies
led to adopt a high-performance concrete
(80 to 100 MPA) and different types of
reinforcement. "Pinched" rings allowed
the lining to follow the curves, by rotating
the position of the ring into 4 possible
locations.
ring of 1.6 m wide for running tunnels.
These six components arrived at the tailend of the backup train and were lifted by
an overhead conveyor. They were moved
by vacuum lifting arms to one of two
idependent vacuum erectors, which
installed them into position. The designed
cycle was about 30 minutes but rings could
be placed in 15 minutes.
In « open » mode, it was possible to erect
segments during excavation, the TBM
pushing against the ground by means of
lateral grippers. In« closed » mode,
excavation and placing of segments could
not be done simultaneously.
Grout is injected afterward in the 15 cm gap
between segments and ground, about 300
m behind the TBM head. It consists of two
components prepared on surface (retarded
sand mortar, aluminous cement slurry). The
10 cm gap between the tail of the
pressurized chamber and the concrete
lining rings was sealed by grease injected
into four rings of metallic brushes inside the
TBM shield.
Breakthrough
Service tunnel
Once the TBMs surveyed after stopping at
100 m distance, a corrective line allowed a
further 50 m drive of the UK TBM, while the
French TBM was being dismantled. The UK
machine was then turned aside on a tight
curve and buried, the remaining being
excavated using hand-held tools (for a
small adit dedicated to the historical
breakthrough), and then roadheaders.
Running tunnels
Junctioning was achieved by diverting each
UK TBM downwards below floor level
(vertical curve, radius 300 m) and driving
the French TBM through to meet the last
UK permanent lining.Ground support
included temporary segments in the invert,
and shotcrete and fibre-glass rockbolts over
the crown. Once the British TBM below the
permanent line, it was backfilled with low
strength concrete (about 2,000 m3). British
lining was then extended through the
remaining skin of the French TBM once
dismantled.
150 km of tunnels have been built, 24 h a
day, on both sides in less than 3.5 years
(4th Jan.1988 to 28th June 1991).
Left : The historical first undersea breakthrough (service tunnel,1st of december 1990). Right: Last breakthrough
(28th of June 1991): French TBM T3 reaches UK cast iron segments,in running tunnel south undersea.
Contrarily to the UK side, the French
Tunnels were lined with bolted precast
concrete segments erected within the
shield. Five main segments (8 tonnes, 40
cm thickness) plus a key formed a complete
TML, THE CHANNEL TUNNEL CONTRACTOR, IS A JOINT VENTURE BETWEEN: Balfour Beatty constructions LTD, Bouygues
S.A., Costain Civil Engineering LTD, Dumez S.A., Société Auxiliaire d'Entreprises S.A., Société Générale d'Entreprises S.A., Spie
Batignolles S.A., Tarmac Constructions LTD, Taylor Woodrow construction holdings LTD, Wimpey major projects LTD.
AMICALE DES BATISSEURS DU TUNNEL SOUS LA MANCHE - 70, rue Mollien 62100 CALAIS
France – TEL/ FAX : +33 (0)3.21.34.06.66 – e-mail batisseurs.tunnel@wanadoo.fr
author Pierre-Jean Pompée, 1995